1. Field of the Invention
The present invention generally relates to a circuit breaker having an overcurrent tripping apparatus. More specifically, this invention is directed to a circuit breaker in which a long-time-delay tripping circuit is selectively operable with a peak value converting circuit and an effective value converting circuit.
2. Description of the Related Art
In a power distributing system, when an accident such as a shortcircuit occurs and a fail current flows through the power distributing system, there is used a system in which a predetermined circuit breaker is made operative to thereby suppress an adverse influence on a power source by the fail current as little as possible. Also, the failed power line can be protected.
For example, as disclosed in Japanese KOKAI (Disclosure) No. 60-32211 (1985) and Japanese U.M. Publication No. 55-29931 (1980), according to such a kind of circuit breaker, when a load make and break (switching) contact is closed and an electric power is supplied from a power source terminal to a corresponding load side terminal through the load make and break contact, and a fail current flows through an AC line, a current transformer detects this fail current at a specific current transforming ratio which is inherent to this current transformer and induces an output current on the secondary winding.
An output signal from a signal converting circuit is supplied to a fail processing circuit. When it is determined that the level of the fail current is at a predetermined level or higher, this level detection signal is input to a timer circuit. The timer circuit executes a predetermined timer operation on the basis of this signal and triggers a gate of a thyristor. Then, the timer circuit energizes a release type overcurrent tripping coil, opens the load make and break contact, and disconnects a fail line from the power distribution system.
As shown in FIG. 1, the timer circuit includes an instantaneous tripping circuit, a short time delay tripping circuit, and a long-time-delay tripping circuit which execute predetermined timer operations when the fail current reaches values in an instantaneous tripping current region, a short time delay tripping current region, and a long time delay tripping current region, respectively (refer to FIG. 6).
FIG. 2 shows an example of a conventional long-time-delay tripping circuit. A detection voltage corresponding to the fail current is converted into its effective value by an effective value converting circuit, which will be explained hereinlater. Thereafter, its output voltage eX is input to a first comparator 35 of a long time delay tripping circuit 170. When the output voltage eX reaches a value which is, for instance, equal to (one time as high as) a reference voltage eY of a reference voltage "eY" setting circuit 37, an output switch 36 in the closed state of the first comparator 35 is opened, thereby enabling a capacitor 38 to be charged. This is because the output voltage eX of the fail current is applied to a voltage/current converter 44 and a predetermined output current I.sub.b is output to the capacitor 38.
Now, assuming that a rated current is set to, e.g., 200 A, the reference voltage eY of the reference voltage "eY" setting circuit 37 is set to, e.g., 0.6 V. When the output voltage eX from the effective value converting circuit reaches 0.6 V, the output switch 36 in the closed state of the first comparator 35 is opened and the charging to the capacitor 38 is started. In this case, a fail current flowing through the AC line at the start of the charging is 240 A.
Between the input voltage eX and the output current I.sub.b of the voltage/current converting circuit 44, there is a predetermined relation such that the output current I.sub.b increases as the input voltage eX increases.
That is, when the input voltage eX is set to, e.g., 0.5 V, the output current I.sub.b is converted into 1 .mu.A, and when the input voltage eX is set to 1 V, the output current I.sub.b is converted into 4 .mu.A.
Therefore, the output voltage eX (1 V) is converted into the current I.sub.b (4 .mu.A) by the voltage/current converting circuit 44. The capacitor 38 is charged by the output current I.sub.b.
When a charging voltage e.sub.1 of the capacitor 38 rises and exceeds an output voltage e.sub.2 of a reference voltage "e.sub.2 " setting circuit 42 of a long time delay operating time, an output signal of a long time delay operating time is output from a second comparator 41, thereby performing the interrupting operation for, e.g., 100 seconds (see FIG. 1).
FIG. 3 shows an example of a typical power distributing system to which the foregoing circuit breaker is connected. In the diagram, PF denotes a power fuse as a protecting device on the high voltage side, and TR indicates a transformer. A circuit breaker B.sub.1 is connected to the secondary winding of the transformer TR. Other circuit breakers B.sub.2 and B.sub.3 are connected to its branch circuit, respectively.
In the power distributing system, if an accident such as a short-circuit occurred in, e.g., a branch circuit of the circuit breaker B.sub.2, only the circuit breaker B.sub.2 operates. None of the upper circuit breaker B.sub.1 of the circuit breaker B.sub.2 and the circuit breaker B.sub.3 of the other branch circuit is made operative. Due to this, the spread of the accident in the power distributing system can be minimized as little as possible, thereby achieving the continuity of the power supply. In this manner, what is called "a selective circuit breaking system" is used.
To accomplish such a selective circuit breaking, as shown in FIG. 4, at least the circuit breakers B.sub.1 to B.sub.3 need to be made adjustable so as not to overlap the operating characteristic curves b.sub.1, b.sub.2, and b.sub.3 of the circuit breakers B.sub.1 to B.sub.3.
On the other hand, in an overload region, the circuit breaker B.sub.1, B.sub.2, or B.sub.3 must obviously operate before fusing the power fuse PF and at the same time, the deterioration of the fuse characteristic of the power fuse PF due to the repetition of the short-circuit accident must be also prevented.
In general, when the operating characteristic curve b.sub.1 of the upper circuit breaker B.sub.1 overlaps a characteristic curve F of the power fuse PF (in the hatched portion in FIG. 4), it is known that the operating characteristic curve b.sub.1 of the circuit breaker B.sub.1 needs to be adjusted to the low current side as shown by a characteristic curve b.sub.12.
When describing the long time delay tripping operating characteristic curve, the adjustment of the operating characteristic curve b.sub.1, b.sub.2, or b.sub.3 of the circuit breaker B.sub.1, B.sub.2, or B.sub.3 can be accomplished by adjusting the current transforming ratio of the foregoing current transformer and the reference voltage eY of the reference voltage "eY" setting circuit 37 in the long time delay tripping circuit 170 in FIG. 2 to a value which is 0.5 to 1 time as large as the maximum set value of eY of the circuit breaker B.sub.1, B.sub.2, or B.sub.3. That is, the long time delay tripping characteristic can be adjusted within a range which is 0.5 to 1 time as large as the maximum rated current of the circuit breaker B.sub.1, B.sub.2, or B.sub.3.
On the other hand, the short time delay tripping characteristic curve of each of the circuit breakers B.sub.1 to B.sub.3 can be also adjusted by means similar to that in the case of the long time delay tripping operating characteristic curve.
However, there is the following problem in the case where only the fail current obtained from the effective value converting circuit is used as an input signal of the long time delay tripping circuit. That is, in a case that the fusing characteristic of the power fuse overlaps the operating characteristic of the relevant circuit breaker due to the saturation characteristic of the current transformer itself, as shown in FIG. 4, it is difficult to adjust this operating characteristic.
In the foregoing conventional system, when the fail current flowing through the AC line lies within a predetermined range where it is 5 to 10 times as large as the rated current, the effective value of the current induced on the secondary winding of the current transformer is saturated as shown in a characteristic curve (J) in FIG. 5, so that a primary current I.sub.a as the fail current and a secondary current I.sub.c (effective value) are not proportional with each other.
Thus, when the fail current falls within the range where it is 5 to 10 times as high as the rated current, the input voltage eX of the comparator 35 of the long time delay tripping circuit 170 is not proportional to the primary current I.sub.a, and therefore, the current which is charged into the capacitor 38 from the voltage/current converting circuit 44 is smaller than the ideal value, so that the charging voltage e.sub.1 of the capacitor 38 does not increase in accordance with a predetermined charging characteristic.
Namely, if the secondary current of the current transformer is influenced by the saturation, the timer operating time of the long time delay tripping circuit 170 is longer than the normal timer operating time, so that the accurate timer operation cannot be finally achieved.
Therefore, when the fail current flowing through the AC line is within a range where it is, for example, 1.2 to 5 times as large as the rated current, assuming that the operating current square time product is set to, e.g., 400 A.times.400 A.times.100 seconds, there is a drawback such that the circuit breaking operation can start only when the operating current square time product is 400 A.times.400 A.times.100 seconds or more, under the condition that the fail current falls within a range where it is 5 to 10 times as large as the rated current.
The present invention has been made to improve the conventional drawbacks and it is an object of the invention to provide a circuit breaker in which even when a fail current flowing through an AC line is sufficiently higher than a rated current, an accurate long time delay operating time can be assured.
It is another object of the invention to provide a circuit breaker in which even in the case of using a current transformer of such a relatively small capacity that tends to saturation, a breaking characteristic of the circuit breaker can be easily adjusted over the entire long time delay region in conjunction with a characteristic of a power fuse employed.